1. Field of the Invention
The present invention relates to improvements to drying cylinders of the type that ar especially encountered in plants for manufacturing cardboard, paper or the like.
2. Discussion of Background Information
Drying cylinders are heated by pressurized steam introduced into their internal cavity. The present invention relates more particularly to the removal of the condensates which form inside these drying cylinders.
The presence of condensates in the cylinder impairs the heat transfer between the steam and the cylindrical casing whose external wall is in contact with the paper or cardboard, whereby the effectiveness of the drying is reduced. Moreover, when the cylinder is stopped, the presence of condensates inside causes localized deformations of the casing due especially to temperature gradients. On restarting the cylinders, these deformations alter the quality of the drying, in a cyclic manner which can, in the manufacture of cardboard, for example, cause internal pasting defects.
Means for recovery of the condensates inside the cylinders do exist. A device described in U.S. Pat. No. 3,449,839 proposes two modes of recovery. A first mode consists in recovering the condensates by means of a fixed scoop maintained at a distance from the internal wall of the cylindrical casing. This device has the disadvantage of being fragile and often not very effective by reason of the difficulty in maintaining a precise clearance between the scoop and the cylindrical casing. A second system consists of suction recovery from a fixed point of the casing of the cylinder. This system has the disadvantage, when the cylinder stops rotating, of leaving the condensates in the cylinder. The suction head is randomly located within the space and becomes totally ineffective if its stop position does not correspond with the bottom point.
Other recovery devices are described especially U.S. Pat. No. 1,575,249 and DE-A-3,143,347. In these two documents, the condensates are recovered by means of orifices pierced into the ends of the cylinder, which orifices are in communication, via channels, with the shaft of the cylinder whereby the condensates escape.
These devices comprise only a limited number of orifices for evacuating the condensates. When the cylinder stops, the condensates can stagnate in the bottom if an evacuating orifice is not located at the bottommost point.
Moreover, these arrangements and, in particular, the orifices pierced in the ends, weaken these ends and the cylinders.